The present invention relates to circuits for supplying power to electrical devices and more particularly to a circuit for preventing a large inrush of current to the electrical device upon the initial application of power to the electrical device and following brief power dropouts.
Certain types of electrically operated equipment include large capacitive elements across which D.C. power is directly applied. D.C. power supplies, for example, often employ large filter capacitors for reducing ripple from rectified A.C. power signals. Upon the initial application of power to this type of equipment, there is a large inrush of current to initially charge the capacitive elements to their normal operating voltage. This large inrush current can damage rectifying diodes and other electrical devices in the current path, such as relays, fuses, etc. It is therefore desirable to include some means for controlling the initial application of power to the equipment so that this large inrush of current is avoided. Patents disclosing current inrush limiting power supplies of this nature include Berger, U.S. Pat. No. 3,787,756, Senger, U.S. Pat. No. 3,617,864, Ellerbeck, U.S. Pat. No. 3,500,174, Cavanaugh, U.S. Pat. No. 3,363,143, Boulanger, et al., U.S. Pat. No. 3,935,511 and Kichak, U.S. Pat. No. 3,996,506.
In a common embodiment, these inrush current limiting circuits control the supply of half or full-wave rectified A.C. power to the filter capacitor included in the power supply which represents the load of the inrush limiting circuit. The inrush limiting circuit includes one or more SCRs for controlling the application of this rectified A.C. signal to the load. Inrush current is limited by controlling the phase angle at which this SCR is triggered in each half cycle of the A.C. power signal. A "soft start" capacitor develops a control voltage across it which is used to establish the triggering point of the SCR in each half cycle of the A.C. signal. Upon the initial application of power to the circuit, the soft start capacitor is fully discharged; the SCR then triggers late in the cycle, permitting only a small portion of the A.C. power to reach the load. Thereafter, however, the soft start capacitor gradually charges, causing the SCR to be triggered at earlier and earlier times in each half cycle of the A.C. power signal. Power is thus gradually applied to the load.
Even with these soft start circuits, however, large inrush currents may still occur following brief power outages. During such power outages, the capacitive element of the load circuit may discharge much more rapidly than the soft start capacitor, hence when power returns the circuit does not "soft start" although the load capacitor is discharged substantially. In this case a harmful inrush current may result.
The aforemention patent to Berger, U.S. Pat. No. 3,787,756, includes a circuit for discharging the soft start capacitor rapidly whenever the power signal is at or near a zero voltage level. Consequently, in the event of a power outage the soft start capacitor will be discharged within several cycles of the A.C. line, and the circuit will soft start upon the return of power thereto. The rate at which the soft start capacitor is discharged in the Berger patent is limited, however, since the circuit will inherently also discharge the capacitor to some extent during the zero crossings of the A.C. power signal. If the rate at which the soft start capacitor is discharged is too great, this discharge between half cycles will cause the circuit to be continually "soft starting". It is therefore necessary to scale the rate at which the soft start capacitor is discharged such that substantial discharge of the soft start capacitor does not occur during the zero crossings. At this rate of discharge, a time interval corresponding to several cycles of the A.C. line may be required to discharge the soft start capacitor and thereby reset the soft start feature. If power returns before the soft start circuit is fully reset, excessive inrush current may result.
The patent to Senger, U.S. Pat. No. 3,617,864, includes a simple mechanical switch which bypasses the soft start capacitor and which may therefore be used to discharge the capacitor and thereby reset the soft start feature. In Senger, however, there is no mechanism for controlling this switch to operate upon the interruption of power. Moreover, since the switch is mechanical in nature, it is unlikely that it could be actuated rapidly enough to properly reset the soft start feature upon brief power outages lasting no more than perhaps several cycles of the A.C. line.